1. Technical Field
The present invention relates to a lockup device used in a fluid type power transmitting device.
2. Background Information
A known example of a fluid type power transmitting device is a torque converter equipped with a lockup device. The lockup device is a mechanism serving to connect mechanically a front cover and a turbine together and is arranged in a space between the front cover and the turbine. The lockup device allows torque to be transmitted directly from the front cover to the turbine.
A conventional lockup device has a piston and a damper mechanism. The piston is arranged such that it can move in an axial direction. When the piston is pushed against the front cover, it moves slidingly along with the front cover. The damper mechanism has a pair of retaining plates, a hub flange, and a plurality of springs that elastically connects the retaining plates and the hub flange together in a rotational direction. The retaining plates are held such that they can elastically deform the springs in the rotational direction and arranged to rotate as an integral unit with the piston. The hub flange is arranged between the pair of retaining plates and fixed to the turbine.
When the piston is pressed against the front cover using oil pressure and a torque is imparted to the front cover, the torque is transmitted to the turbine through the piston and the damper mechanism. During the torque transmission, torsional vibrations are absorbed and damped by the damper mechanism (e.g., see Japanese Patent Publication Number 3542186).
The lockup device presented in Japanese Patent Publication Number 3542186 has an inertia member provided in a torque transmission path to lower a resonance frequency to a frequency lower than a practical rotational speed and improve the vibration damping performance.
Meanwhile, in recent years, there is a demand to expand a lockup region in which the lockup device is used to transmit power into a region of lower traveling speeds in order to improve the fuel efficiency of a vehicle.
However, the lockup device presented in Japanese Patent Publication Number 3542186 is designed to be used only during normal travel of a vehicle and cannot easily secure an acceptable vibration damping performance at, for example, low traveling speeds. That is, the conventional lockup device cannot accommodate an expanded lockup region and it is difficult to improve the fuel efficiency of a vehicle using the conventional lockup device.